The major drawback to the use of fuel cells as electric generators and auxiliary power units (APUs) in commercial and passenger vehicles is their inability to directly use readily available transportation fuels. Gasoline, diesel or jet fuels are ideal for fuel cells because of their high energy density, ready availability, safety and ease of storage. These fuels must be converted to a synthesis gas feed using a reformer (e.g., steam reformer, partial oxidizer, or catalytic partial oxidizer) for use in fuel cells. In addition, all fuel cells require an essentially sulfur-free feed stream in order to prevent poisoning of fuel cell anode catalyst, which requires effective desulfurization of either the liquid fuel or the synthesis gas feed. Even the more robust high temperature solid oxide fuel cells (SOFCs) are poisoned with low levels of sulfur contaminants. Although the U.S. Environmental Protection Agency (EPA) has new regulations in place that mandate refineries to reduce sulfur levels of transportation fuels down to 30 parts per million on weight basis (ppmw) in gasoline and 15 ppmw in road diesel; even with meeting these reduction levels, sulfur concentrations in transportation fuels will still exceed the levels tolerable by state of the art SOFCs, about 100 parts per billion on volume basis (ppbv), preferably 10 ppbv or less. In the reforming step, the organic sulfur species in the fuel (e.g., ranging from thiols to dimethyldibenzothiopehenes) are converted mainly to hydrogen sulfide (H2S) and carbonyl sulfide (COS), and contaminate the reformate gas stream. Even when using Ultra Low Sulfur Diesel (ULSD) fuel and with the dilution in the fuel reformer (due to the air intake, addition of steam or anode tail gas recycle), the reformate gas contains more than 3 parts per million on volume basis (ppmv) sulfur which is well beyond the desired range of the state-of-the-art fuel cells. Similar to the transportation fuels, the sulfur in other hydrocarbon feeds, such as natural gas and liquefied petroleum gas suitable for stationary applications also must be reduced to low levels (because natural gas is colorless, odorless, and tasteless, sulfur-bearing odorants such as mercaptans or dimethyl sulfidorganic is added before distribution to give it a distinct odor that serves as a safety device by allowing it to be detected in case of a leak. In being reformed these organic sulfur compounds also produce H2S).
While sorbent technologies are currently available for removing sulfur from reformate gas, they are not suitable for use at the very high temperature needed for feed gas to SOFCs (greater than 500° C. and typically 700-800° C.). The currently available sorbents exhibit higher sulfur capacity, removal rate and stability at more moderate temperatures below about 500° C. For example, most conventional post-reformate treatment systems use a metal oxide sorbent (e.g., zinc oxide and its derivatives) that covalently binds sulfur. While, such sorbents can be effectively used up to 500° C., sulfur slippage from these sorbents increases significantly in the 700-800° C. range allowing sulfur concentration in the feed gas which well exceed the levels tolerable by the fuel cells. For example, the equilibrium H2S concentration over the ZnO sorbent is calculated as 5.6 ppmv and 15.7 ppmv at 700 and 800° C., respectively, using a gas composition representative of hot reformate gas. Therefore, conventional metal oxide sorbents will only be useful in such applications, if the temperature of reformate gas is reduced to 500° C. or less to carry out the desulfurization. For SOFC systems, after removal of sulfur, the desulfurized gas must be reheated before being introduced into the fuel cell (typically operating at 700-800° C.). The added steps of reformate gas cooling and re-heating reduce the overall energy conversion efficiency and require the use of expensive heat exchangers, which increases system cost, volume and complexity.
A number of sorbents for sulfur removal are known in the art. For example, D. A. Gribble, S. A. Rolfe and M. V. Mundschau (2009) International Pittsburgh Coal Conference provide a review of perovskite sorbents for warm-gas removal of sulfur. Song, Chunshan (2003) An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catalysis Today 86:211-263 provides a review of materials for desulfurization. at lower temperatures.
Wang, Z., Flytzani-Stephanopoulos, M. “Cerium Oxide-Based Sorbents for Regenerative Hot Reformate Gas Desulfurization” (2005) Energy & Fuels, 19: 2089-2097 reports cerium oxide based sorbents for regenerative hot reformate Gas Desulfurization. More specifically the reference relates to lanthanum- or copper-containing cerium oxide sorbents for desulfurization of simulated reformate at 650 and 800° C. US published patent application 20080267848, published Oct. 30, 2008 reports an apparatus and methods for non-regenerative and regenerative hot gas sulfurization using a doped cerium oxide sorbent.
A number of references relate to the use of manganese ore and related species as sorbents for high temperature desulfurization. Bakker, J. W., Kapteijn, H., Moulijn, J. A., “A high capacity manganese-based sorbent for regenerative high temperature desulfurization with direct sulfur production: Conceptual process application to coal gas cleaning,” Chemical Engineering Journal, 96, (2003) 223-235 reports bulk removal of sulfur from dry coal gas at temperatures ranging from 400 to 1000° C., with optimum capacity reported at 827 to 927° C. using a sorbent described as crystalline MnAl2O4, with a small amount of disperse MnO and an amorphous Mn—Al—O phase. Sulfur reduction from 1% H2S to less than the detection limit of 5 ppm was reported. Ben-Slimane, R., Hepworth, M. T., “Desulfurization of Hot Coal-Derived Fuel Gases with Manganese-Based Regenerable Sorbents. 1. Loading (Sulfidation) Tests,” Energy & Fuels, 8, (1994a) 1175-1183 and Ben-Slimane, R., Hepworth, M. T., “Desulfurization of Hot Coal-Derived Fuel Gases with Manganese-Based Regenerable Sorbents. 2. Regeneration and Multicycle Tests,” Energy & Fuels, 8, (1994b) 1184-1191 report sulfur sorbents based on manganese ore and more specifically a combination of manganese carbonate, alundum, and bentonite, or a combination of manganese ore, alundum and dextrin for hot fuel gases at temperatures ranging from 750 to 1000° C. Certain sorbents are said to be highly-effective, inexpensive, and regenerable. Yoon, Y., Kim, M. W., Yoon, Y. S., Kim, S. H., “A kinetic study on medium temperature desulfurization using a natural manganese ore,” Chemical Eng. Sci., 58, (2003) 2079-2087 reports the use of natural manganese ore at temperatures ranging from 400 to 800° C. for sulfur removal. NiO addition was reported to improve sulfidation capacity.
Liang, B., Korbee, R., Gerritsen, A. W., Van den Bleek, C. M., “Effect of manganese content on the properties of high temperature regenerative H2S acceptor,” Fuel, 78, (1999) 319-325 report Mn/γ-Al2O3 acceptor for high temperature, regenerative H2S removal was prepared by repeated impregnation. For a sample with a manganese content of 34 wt %, a sulfur capacity of about 22 wt % was reported for sulfidation at 850° C.
L. Alonso, J. M. Palacios, and R. Moliner (2001) The Performance of Some ZnO-Based Regenerable Sorbents in Hot Coal Gas Desulfurization Long-Term Tests Using Graphite as a Pore-Modifier Additive, Energy Fuels 15(6):1396-1402 report ZnO based sorbents for sulfur removal.
Suk Yong Jung, Soo Jae Lee, Tae Jin Lee, Chong Kul Ryu, Jae Chang Kim (2006) H2S removal and regeneration properties of Zn—Al-based sorbents promoted with various promoters, Catal. Today 111 (3-4) 217-222 and Suk Yong Jung, Soo Jae Lee, Jung Je Park, Soo Chool Lee, Hee Kwon Jun, Tae Jin Lee, Chong Kul Ryu, Jae Chang Kim (2008) The simultaneous removal of hydrogen sulfide and ammonia over zinc-based dry sorbent supported on alumina, Separation Purif. Technol. 63:297-302 report certain zinc-based sulfur sorbents and report the use of NiO as a promoter in such sorbents.
Siriwardane, R. V., Todd Gardner, James A. Poston, Jr. and Edward P. Fisher, Spectroscopic Characterization of Nickel Containing Desulfurization sorbents, Ind. Eng. Chem. Res. 39 (2000) 1106-1110 reports tests of sorbent pellets with simulated coal-derived hot fuel gas containing H2S at 538° C.
In certain embodiments of the invention, the nickel phase is deposited by a method that is believed to generate nickel hydroxide and oxide nanoparticles. The following references relate to generation of nickel hydroxide powders or nanoparticles: Akinc, M., Jongen, N., Lemaître, J., Hofmann, H., “Synthesis of nickel hydroxide powders by urea decomposition,” J. European Ceramic Soc., 18, (11), (1998) 1559-1564; Jayalakshmi, M., Venugopal, N., Ramachandra Reddy, B., Mohan Rao, M., “Optimum conditions to prepare high yield, phase pure α-Ni(OH)2 nanoparticles by urea hydrolysis and electrochemical ageing in alkali solutions,” Journal of Power Sources, 150, (2005) 272-275; Li, J., Yan, R., Xiao, B., Liang, D. T., Lee, D. H., “Preparation of Nano-NiO Particles and Evaluation of Their Catalytic Activity in Pyrolyzing Biomass Components,” Energy & Fuels, 22, (2008) 16-23; Soler-Illia, G. J. A. A., Jobbágy, M., Regazzoni, A. E., Blesa, M. A., Argentina, “Synthesis of Nickel Hydroxide by Homogeneous Alkalinization. Precipitation Mechanism,” Chem. Mater., 11 (11), (1999) 3140-3146; and Liang, Z. H., Zhu, J. Y., Hu, X. L., “-Nickel Hydroxide Nanosheets and Their Thermal Decomposition to Nickel Oxide Nanosheets,” J. Phys. Chem. B, 108, (2004) 3488-3491.
There is a need in the art for sorbents exhibiting high capacity, and efficient sulfur removal to levels preferably down to 10 ppbv or less from reformate gas at temperature greater than 500° C. and preferably at temperatures ranging from 700 to 800° C.